Unlike typical simian immunodeficiency virus infection of macaques, which induces an AIDS-like syndrome, SIVsmmPBj infection in pig-tailed macaques induces an acutely lethal disease. Our group has been involved in elucidating the genetic and biologic mechanisms involved in the induction of this unusual disease. This work will assist in further defining the roles of SIV (HIV) gene products in pathogenesis. In addition, analysis of the mechanisms of early pathogenesis of this virus may enhance our understanding of early HIV/host interactions. Previous work has shown that multiple viral determinants contribute to the pathogenesis of disease. These determinants are located in gag, nef, and the central regulatory region. To further define the contribution of nef to acute pathogenesis, mutations in the 17th and 18th amino acids were generated. Viruses containing these mutations did not induce proliferation of PBMC in vitro, and did not induce acutely lethal disease in vivo in three inoculated pig-tailed macaques. Thus, the critical area of nef essential for acute pathogenesis has been defined. We have started a time-course experiment to examine the in vivo localization of virus following SIVsmmPBj inoculation. Six pig-tailed macaques were inoculated with SIVsmmPBj. One animal was euthanatized per day and tissues were harvested for localization of virus. In vitro cell-cell titration experiments showed that by day 2 after infection, a number of tissues contained large amounts of virus (up to 103 TCID50/106 cells). These studies will continue to examine how the virus replicates and disseminates following infection. In collaboration with Dr. Steve Dewhurst, we have examined the levels of apoptosis in tissues from pig-tailed macaques infected with SIVsmmPBj. These studies have shown that increased apoptosis is observed in gut tissues, where disease is manifest, but not in other tissues. In addition, this collaboration has resulted in the findings tha t SIVsmmPBj can increase the levels of the gut-associated integrin ?E?7 on lymphocytes both in vitro and in vivo, supporting our hypothesis that this disease is due, in part, to the homing of infected lymphocytes to the gut area. Subsequent analyses will focus on using the time course animals to examine both lymphocyte homing and apoptosis.